1. Field of the Invention
The present invention relates generally to field of drilling or boring and, more specifically, to an apparatus which may be used to ream, underream or backream a borehole such that the borehole diameter is increased by a desired amount up to several multiples of the original diameter.
2. Background Information
Conventional drilling for oil, gas or groundwater is typically done by boring a hole having a diameter on the order of 6 to 10 inches. The borehole is extended to a sufficient depth to reach one or more geological structures of interest (i.e., pay zones) which are known or believed to exist.
It is well known that large quantities of oil, gas and groundwater are reachable but have remained largely undeveloped because the geological formations which contain these resources exhibit little or no permeability and very low porosities. An example of such a formation is found in southern Mississippi and adjacent areas of Louisiana. The formation consists essentially of oil saturated shale with dissolved natural gas, at approximate depths of 10,000 to 14,000 feet, and having net thickness on the order of 25 to 250 feet. A borehole of conventional size, drilled vertically into the shale and completed using conventional methods, may only produce on the order of 1 to 5 barrels of oil per day. In terms of historical market prices for crude oil, such low volume renders production uneconomical in view of the costs of drilling, completion and extraction.
One conventional approach to this problem involves injection of various liquids, laden with solid particles of appropriate size and concentration, under high pressure to create fractures and channels which enable the resource to exit the formation and enter the borehole. However, high-pressure fracturing in a pay zone will largely fail if the zone is not underlain and overlain by dense, impermeable strata. Otherwise, the fracturing elements, including the high pressure, are dissipated and ineffective.
In other geological settings, very thin strata of very fine granular sediments are sandwiched between substantially thicker strata of impermeable rock. These very thin pays zones are extremely difficult to target by perforations through casing in conventional vertical boreholes. Further, such thin pay zones are not always susceptible to improvement of deliverability by induced high-pressure fracturing or extended reach or horizontal boreholes.